Physical exercise can sometimes be used to elicit and study maximal or near maximal responses of various metabolic systems. It is proposed that the relative and absolute roles played by lactic acid and related substances, such as glucose, branched chain amino acids, and other glycolytic and gluconeogenic intermediates in the metabolic adjustments to exercise be evaluated. Initially, the effect of variations in metabolic rate on the oxidations of infused 14C lactate, glucose, pyruvate, alanine, glutamate, leucine, isoleucine, aspartate, glutamine, and glycine will be evaluated in rats. Subsequently, the effect of variations in metabolic rate (ie., exercises at 40 and 80% of V02 max) on lactate and glucose turnover will be evaluated by contemporary isotope (14C) infusion techniques. These first sets of experiments will lay the basis for kinetic tracer studies to allow precise determination of the pathways of lactate metabolism during exercise and the immediate post-exercise (02 debt) period. In these experiments, metabolites present in various tissues (heart, liver, kidney, small intestine, red and white skeletal muscle) will be separated and quantified by a series of techniques including: ion-retardation column chromatography, two dimensional paper chromatography, autoradiography, semi-automated Geiger counting, and enzymatic assays. These data, along with reliable estimates of the masses of various tissues, will allow carbon flow within and between tissues to be mapped. These studies will lay the bases for more sophisticated studies utilizing specifically and doubly labeled tracers to elaborate the controls of carbon flow during exercise.